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University  of  Illinois  Bulletin 

Vol.  VI.  JULY  14.  1909  No.  39 

[Entered    February    14,    1902,    at    Urbana,    Illinois,    as   second-class   matter    under 
Act   of  Congress   of  July    16,    1894.] 


BULLETIN  No.  11 

DEPARTMENT  OF  CERAMICS 

C.    W.    ROLFE.    D,r«tor 


A  Cheap  Enamel  for 
Stoneware 

PART  II. 


By 

RAY  THOMAS  STULL 


1908—1909 


published  fortnightly  by  the  university 


A  CHEAP  ENAMEL  FOR  STONEWARE. 

Part  II. 

R.  T.  Stull,  Urbana,  Illinois. 

The  last  contribution  of  "A  Cheap  Enamel  for  Stone- 
ware," in  Vol.  X,  was  sent  in  for  publication  three  days 
after  the  last  trials  came  from  the  kiln.  The  most  promis- 
ing enamels  of  the  work  done  up  to  that  time  were  the 
members  of  Series  IX,  and  the  best  one  of  the  series  was 
No.  35. 

As  has  been  noted,  no  craze  marks  were  observed  on 
Xo.  35,  when  taken  from  the  kiln.  After  three  weeks'  time 
a  craze  mark  appeared  on  the  bottom  of  one  trial.  Crazing 
has  been  going  on  gradually  until  now  the  trials  are 
completely  checkered  with  craze  marks.  The  author  is  of 
the  opinion  that  the  crazing  is  due  to  the  softness  of  the 
body  rather  than  to  the  glaze  itself,  since  the  trials  were 
burned  at  cone  01. 

series  x. 

The  maturing  temperature  of  Xo.  35  is  altogether  too 
low  for  the  stoneware  body  and  too  high  for  other  .types 
of  ware,  notably  those  maturing  at  cone  04.  Hence  it  was 
thought  advisable  to  construct  a  series  starting  with  a 
glaze  of  the  same  type  as  Xo.  35,  but  which  should  be  more 
fusible.  The  RO,  A1203  and  P20 ,,  which  are  the  same  as 
those  in  Xo.  35,  are  kept  constant  throughout  the  series, 
the  two  variable  members  being  Si02  and  B203. 

Each  succeeding  member  has  the  B203  decreased  one 
tenth  and  the  SiQ2  increased  accordingly.    Two  fritts  were 


A    CHEAP    ENAMEL   FOR    STONEWARE. 


made  for  Series  X,  which  have  the  following  formulae  and 
batch  weights : 


Fritt  G. 


Formula 


Batch    weights 


S571   CaO]  f2.3  Si02       Bone   Ash 

I- .357   Al2Osi  Zinc   Oxide 

1429  ZnOj  L    -2857P205  N.  C.  Kaolin 

Flint 


30.75 

4.03 

32.09 

33.13 


Fritt  H. 


Formula 


Batch   weights 


.25   K20]  f2.125   SiO= 

J- .5625  AlsOs-l     .250  P2O5 

.75   CaO  J  [1.250   B.Os 


Feldspar 
Bone  Ash 
N.  C.  Kaolin 
Boracic  Acid 


30. SO 
17.09 
17.83 
34.28 


Fritt  G  was  calcined  in  a  Hinted  sagger  at  cone  8. 
Fritt  H  was  fused  in  a  Hinted  crucible  and  quenched  in 
water.  Both  fritts  were  ground  dry  to  a  fine  powder  ready 
for  weighing. 

Glazes. 


Formulae 


Batch   weights 


V 

S3  -• 

2 

0 
u 

0 

6 
< 

6 

d 

0 

PQ 

CuUh 

O 

'/ 
O 
u 

N 

- 
'5c.  ~ 

u.  — 

0  0 

1/   cd 

- 

36    |    . 

0 

.6    1 

.2 

.55 

2.3 

.2    |    1.0 

81.90 

4.45 

7.07 

6.58 

37    !   • 

2.4 

"     |'    .9 

3.06 

5.28 

73.91 

4.24 

7.09 

6.42 

38   I   ' 

2.5 

"    1     .8 

6.14 

10.60 

65. S8 

4.02 

7.11 

6.25 

39    |    ' 

" 

2.6 

.7 

9.24|l5.94 

57.80 

3.81 

7.13 

6.08 

40    |    ' 

" 

2.7 

"     1     .6 

12.35l23.31 

49.68 

3.59 

7.15 

5.92 

41    I    ' 

" 

" 

2.S 

" 

.5 

15.48l26.71 

41.51 

3.38 

7.17 

5 . 7  5 

42    I   ' 

" 

a 

" 

2.9 

.4 

18.63|32.14 

33.30 

3.16 

7.19 

5.58 

43    1    ' 

" 

" 

3.0 

.3 

21.79137.60 

25.04 

2.94 

7.21 

5.42 

44    |    ' 

" 

" 

3.1 

.2 

24.79|43.09 

16.74 

2.72 

7.23 

5.25 

45    1    ' 

' 

" 

" 

" 

3.2 

.1 

28.17|48.60 

S.39 

2.51 

7.25 

5.0S 

46    |    ' 

"  ! 

" 

"  1 

3.3 

" 

.0 

31.39l54.16 

2.28 

7.27 

4.90 

A    CHEAP    ENAMEL    FOR    STONEWARE.  5 

These  glazes  were  applied  to  bone  dry  stoneware 
crocks  but  flaked  so  badly  on  drying  that  the  series  was 
abandoned.  A  few  subsequent  dips  of  the  two  extremes 
of  this  series  (Nos.  36  and  4G )  worked  very  satisfactorily 
on  biscuit.  No  further  investigation  was  made  into  the 
working  qualities  of  the  members  of  this  series  on  biscuit 
since  the  work  is  mainly  along  the  line  of  single  fire 
enamels. 

SERIES  XI. 

Series  X  was  modified  in  series  XI  to  overcome  flak- 
ing. In  this  series  the  same  formulae  are  used  as  in 
scries  X,  the  difference  being  that  in  series  XI  the  raw 
clay  is  increased  from  0.1  to  0.15  and  Tennessee  ball  clay 
No.  •">  is  used  in  place  of  Georgia  kaolin.  This  required 
the  making  of  two  new  frit ts  (I  and  J),  since  the  excess 
clay  above  <U   in  series  X  was  fritted. 

Fritt   I. 

Formula  Batch  weights 

.5  K-O]                       f 3.0  SiO-  Brandywine  Feldspar  44.2S 

.5     AUCM  Zinc  Oxide  6.44 

.5  ZnO  J                        (  2.5   B:0;;  Boracic  Acid  49.28 
Combined   weight   494. 

Fritt  J. 

Formula  Batch    weights 

("1.3J3   SiO,        Bone  Ash  45.0o 

1.0   CaO     .Zy3  AUCM  X.   C.   Kaolin  37.50 

[    .3^   P=0.,        Flint  17.44 

The  first  one  was  drop  fritted  while  the  latter  was 
calcined  to  cone  8  in  a  fiinted  sagger.  Both  were  reduced 
to  powder  and  used  in  this  form  in  weighing  up  the 
glazes. 


A    CHEAP    ENAMEL    FOR    STONEWARE. 


Batch  Weights  Series  XI    (Having  the  same  succes- 
sive formulae  as  Series  X). 


N 

03 

5 

to 

T3  in 
V.-0 

Fritt    I 

Fritt  J 

."2 
O 

o 
a 

N 

Tenn.   Ball 
Clay   No.  3 

E 

47 

53.89 
48.63 

35.56 
35.66 

10.55 
10.58 

48 

3.05 

0.44 

1.64 

49 

6.11 

43.35 

35.75 

0.89 

10.61 

3.29 

50 

9.19 

38.03 

35 .  85 

1.34 

10.64 

4.95 

51 

12.28 

32.69 

35.95 

1.79 

10.67 

6.62 

52 

15.40 

27.32 

36.05 

2  ^4 

10.70 

8.29 

53 

18.53 

21.91 

36.15 

2.70 

10.73 

9.98 

54 

21.68 

16.48 

36.25 

3.15 

10.76 

11.68 

55 

24.85 

11.02 

36.35 

3.61 

10-79 

13.38 

56 

28 . 03 

5.52 

36.46 

4.08 

10.82 

15.09 

57 

31.23 

36.56 

4.54 

10.85 

16.82 

Three  burns  were  made  of  the  members  of  this  series. 
The  first  four  members  were  burned  to  cone  04,  the  first 
six  to  cone  01,  and  all  were  burned  to  cone  6. 


Cone  04  Burn. 

No.  47  in  this  burn  appeared  to  be  well  matured  and 
glossy.  It  was  not  quite  so  opaque  nor  light  colored  as 
No.  35  (see  p.  239  and  p.  241,  Vol.  X,  Trans.  A.C.S.).  A 
few  small  bubbles  appear  on  the  exposed  rim.  The  glaze 
crazes  some  on  the  soft  buff  body. 

No.  4S  is  the  best  glaze  of  the  burn,  though  it  has  a 
very  slight  tendency  to  pull  away  from  the  rim.  It  is 
nearly  as  opaque  and  light  in  color  as  No.  35.  It  crazes 
some. 

No.  49  (having  the  same  formula  as  No.  35)  is  under- 
fired,  being  a  little  dull  in  texture.  It  is  opaque  and 
nearly  white,  crazes. 

No.  50  is  decidedly  underfilled,  having  the  appearance 
of  highly  vitrified  porcelain.  Badly  crazed.  In  two  places 
the  glaze  has  loosened  and  stands  out  from  the  body. 


A    CHEAP   ENAMEL   FOR    STONEWARE. 


Gone  01  Burn. 

No.  47  is  glossy,  but  the  opacity  has  decreased,  giving 
a  translucent  glaze.  Bubbling  has  also  increased,  crazing 
decreased. 

The  opacity  of  No.  48  has  also  diminished.  The  glaze 
in  this  burn  is  otherwise  good. 

No.  49  is  opaque  but  not  quite  so  light  colored  as  No. 
35.  A  slight  tendency  to  flake  appears.  The  glaze  shows 
a  few  craze  marks. 

No.  50  is  more  opaque  than  49  though  not  quite  so 
glossy.     Crazes  badly. 

Nos.  51  and  52  are  underfired  and  crazed. 

Cone  6  Burn. 

In  this  burn  all  glazes  show  small  bubbles  up  to  No. 
55.  No.  47  is  bubbled  the  worst,  having  a  frothlike  ap- 
pearance. Bubbling  decreases  towards  No.  55.  No.  55 
has  a  pitted  appearance.  These  glazes  are  undoubtedly 
overtired. 

No.  56  is  well  matured  and  shows  no  bad  defects.  Its 
whiteness  is  a  little  better  than  that  of  a  good  bristol. 

No.  57  is  a  little  whiter  than  5G  though  not  brilliant 
enough  to  be  a  good  glaze.     It  would  be  better  at  cone  7. 

Conclusions  on  Series  XI. 

The  glazes  of  this  series  are  capable  of  producing  glossy 
opaque  enamels  when  given  the  right  heat  treatment.  If 
a  little  underfired,  they  are  dry  and  dull.  When  slightly 
overflred,  they  lose  opacity  and  begin  to  develop  bubbling. 
The  range  of  temperature  is  very  short,  not  over  1^  cones 
at  most.  For  this  reason  they  are  not  commercially  valu- 
able. 


8  A    CHEAP    ENAMEL    FOR    STONEWARE. 

The  writer's  practical  experience  in  glazes  in  which 
magnesite  was  used,  brought  out  the  fact  that  MgO  pos- 
sessed strong  opacifying  properties.  Tin  enamels  con- 
taining 0.25  MgO  required  less  tin  oxide  in  order  to  pro- 
duce the  same  degree  of  opacity  than  those  containing  no 
MgO.  It  was  also  observed  in  testing  feldspars  that  soda 
feldspar  of  a  pure  grade  fused  to  a  whiter  and  more  opaque 
bead  than  potash  spar.  The  crystalline  form  of  soda  spar 
generally  occurs  as  a  porcelain  white  mineral  called 
Albite;  potash  spar,  or  Orthoclase,  on  the  other  hand,  is 
more  translucent  and  dark  in  color. 

If  it  is  true  concerning  bases  belonging  to  the  same 
chemical  group  or  family  that  those  of  low  molecular 
weight  possess  a  lower  degree  of  solubility  in  a  silicate 
fusion  and  a  higher  velocity  of  crystallization  than  bases 
of  high  molecular  weight,  then  less  molecular  quantities 
of  Na20  and  MgO  would  be  required  to  produce  "super- 
saturation"  and  "incipient  crystallization"  than  the  bases 
of  their  corresponding  groups  having  higher  molecular 
weights,  namely  K20  and  CaO. 

Acting  upon  the  above  suggestions,  series  XII  and 
XIII  were  constructed.  It  will  be  noted  that  the  members 
of  these  series  are  glazes  of  the  bristol  type,  in  which 
Na,0  is  substituted  for  K20  and  MgO  replaces  CaO.  In 
these  two  series  the  RO  is  kept  constant. 

SERIES  XII. 


Formulae 

Batch  wciglits 

o 

N 

- 

3 

q 
1 

o 
be 

C 

N 

6 

< 

5 

v  be 

u 
"y. 

u 
N 

-   _■ 
HU 

- 

58 
59 
60 
61 
62 
63 

1    f/z 

f/z 

f/z 

.f/z 

" 

2.2 

2.5 
2.8 
3.1 
3.4 
3.7 

I    68.37 
63.88 
59.94 

56.45 
53.34 

|     50.56 

10.96 
10.24 
9.60 
9.05 
8.55 
S.10 

10.57)     10.10 

9.87]       9.4:i 
9.26|       8.85 
8.721       S.33 
8.25|       7.S8 
7.82)       7.47 

6. 58 
12.35 
17.45 
21. 9S 
26.05 

A    CHEAP    ENAMEL    FOR    STONEWARE. 


SERIES    XIII. 


Formulae 

B 

atch  zveights 

ct 

"y. 

o 

Ball 

No.  1 

N 

.-t 

O 

0 

O 

■j. 

Z 
N 

Z 
< 

o 

■jf. 

—  — 

O    u 

-5S 

o 
N 

Tenn 
Clay 

g 

64 

■  3/ 

•  3/3 

■3/3 

.5 

2.3/ 

64.06 

10.27 

9.90 

15.77 

(15 

" 

•• 

*' 

2.6/3 

6(1.10 

9.63 

9.29 

14.79 

6.19 

lili 

" 

•• 

" 

2.9/3 

56.59 

9.07 

S.75 

13.93 

11.66 

67 

" 

•' 

" 

3.2/ 

53.48 

8.57 

8.26 

13.16 

16.53 

68 

" 

" 

'• 

3.5/ 

50.68 

S.12 

7.83 

12.48 

20.89 

69 

" 

" 

3.8/ 

48.17 

7.72 

7.44 

11.86 

24.S1 

These  glazes  were  applied  to  bone  dry  stoneware  trials 
and  fired  to  cone  8.  They  were  taken  from  the  kiln  while 
still  quite  hot.  None  of  them  show  any  signs  of  crazing. 
There  is  very  little  difference  in  the  whiteness  of  the  mem- 
bers of  the  two  series.  All  are  as  opaque  and  white  as  a 
good  tin  enamel.  Nos.  58  and  04  are  a  little  dull.  Glos 
siness  increases  with  increase  in  Si02.  The  glossiness  of 
Xos.  CO  to  63  and  (Hi  to  69  are  very  good  and  in  this  re- 
spect will  compare  favorably  with  the  majority  of  stone- 
ware glazes  on  the  market  and  are  far  superior  to  them 
in  whiteness.  All  five  pieces  of  No.  (52  are  especially  good. 
All  members  of  Series  XII  have  a  slight  tendency  to  bead 
where  the  glaze  is  too  thick.  The  glazes  do  not  show  any 
tendency  to  pull  away  from  the  corners  and  edges.  The 
small  beading  tendency  can  be  overcome  by  the  addition 
of  borax. 

In  Scries  XIII  in  which  the  A1203  is  increased  the 
beading  has  increased  a  little.  The  glazes  do  not  appear 
to  be  quite  so  fusible.     Otherwise  they  are  good. 

In  order  to  compare  the  opacifying  effect  of  MgO  with 
that  of  ZnO,  the  following  series  was  prepared  by  the  class 
in  glaze  making  in  their  regular  laboratory  work.  In  this 
series  I  Series  XIV  i  a  glaze  of  the  Bristol  type  was  selected 
for  the  first  member.  In  each  succeeding  one  0.1  MgO 
replaces  0.1  ZnO,  all  other  members  remaining  constant. 


10 


A    CHEAP    ENAMEL   FOR    STONEWARE. 


SERIES  XIV. 


Formulae 

Batch  weights 

—  , 

u 

<u 

T3 

re     . 

re 

be 

PQ  o 

C     "5 

'S 

A  a 

c 

.fc 

rt  ¥• 

o 

4J 
N 
03 

O 

o 

re 

O 

o 

be 

6 

6 

C3     c/1 

re  -3 

O  *0J 

.a 

5.3 

'I  to 

1-  JS 

u 

■g 

3 

W 

U 

N 

§ 

< 

c/5 

D^fc 

^ 

HU 

OS 

N 

E 

H-l 

.3 

.2 

.5 

.0 

.5 

4.0 

1G7.1 

20. 0| 

51.6 

|   40.5 

128.0 

H-2 

" 

.4 

.1 

" 

" 

" 

" 

" 

8.4 

32.4 

" 

H-3 

" 

.3 

.2 

" 

" 

" 

"   1 

" 

16.8 

24.3 

" 

H-4 

" 

.2 

.3 

" 

" 

" 

"   1 

" 

25.2 

16.2 

" 

H-5 

" 

" 

.1 

.4 

" 

" 

" 

"  ! 

" 

33.6 

8.1 

" 

H-6 

" 

" 

.0 

.5 

" 

" 

" 

"  1 

" 

42.0 

" 

These  glazes  were  applied  to  small  bone  dry  stone- 
ware crocks  and  fired  to  corie  7  in  25  hours. 

There  is  but  little  difference  in  the  appearance  of  the 
members  of  this  series.  If  anything,  H-6  is  a  little  the 
whitest  and  smoothest  in  texture.  No  bad  defects  appear. 
The  conclusion  which  we  may  draw  from  the  appearance 
of  the  trials  is  that  MgO  is  at  least  equally  as  good  an 
opacifier  as  the  same  molecular  quantity  of  ZnO. 

DISCUSSION. 

Mr.  Stull:  Referring  to  the  trials  will  say  that  I 
have  brought  only  two,  No.  35  representing  the  most 
promising  enamel  of  last  year's  work  and  No.  62  which 
is  the  most  promising  one  to  date.  You  will  notice  that 
No.  35  is  a  sort  of  ivory  color  and  matures  altogether  too 
low  for  stoneware.  The  trials  were  nested,  and  happened 
to  touch  at  this  point  (indicating)  where  a  bubble  formed, 
hence  this  bubble  near  the  rim  is  not  really  a  defect  of  the 
glaze  in  itself.  You  will  notice  that  the  enamel  is  badly 
crazed,  and  has  a  little  tendency  to  flake  though  not  bad 
enough  to  be  serious.  No.  62  is  a  member  from  Series 
XII.  You  will  notice  in  the  bottom  where  the  glaze  is 
thick,  it  beaded  in  one  spot  and  partially  healed  over 
again.  Those  who  have  had  experience  in  Bristol  glazes 
will  agree  with  me  that  the  beading  is  not  bad,  since  no 
"dope"  was  added  to  these  glazes  to  prevent  it. 


A    CHEAP    ENAMEL    FOR    STONEWARE.  11 

There  is  a  great  field  open  for  investigation  to  de- 
termine the  opacifying  effects  of  different  materials.  We 
have  evidence  that  magnesia  is  a  good  opacifier;  a  good 
grade  of  magnesite  when  properly  used  in  conjunction 
with  zinc  oxide  will  give  whiter  and  glossier  effects  than 
the  regular  type  of  Bristol  glaze. 

Mr.  Purdy:  Those  of  us  who  have  had  experience 
along  this  line  note  with  interest  the  discussion  of  the 
opacifying  effect  of  Na20  as  against  K20,  and  Mgo  as 
against  CaO,  in  conjunction  with  zinc. 

Mr.  Jackson :  In  regard  to  the  trial  passed  around, 
I  notice  two  temperatures  mentioned,  for  instance,  cone 
01  and  cone  8.  I  am  not  very  familiar  with  the  stoneware 
business,  but  I  would  like  to  ask  whether  it  is  customary 
to  have  two  fires,  a  biscuit  and  glost?  Do  I  understand 
that  the  piece  going  around  had  a  previous  biscuit  fire? 

Mr.  Stull:     No,  sir. 

Mr.  Purdy:  There  is  only  one  part  of  the  country 
that  I  know  of  where  they  use  two  fires,  and  that  is  in  the 
West  where  they  have  clays  of  enormous  shrinkage. 

Mr.  Jackson:  Is  it  practicable  and  customary  to 
make  stoneware  as  low  as  01? 

Mr.  Purdy:  It  is  not  customary;  it  might  be  prac- 
ticable. 

Mr.  Stull:  I  am  sure  there  is  no  true  stoneware  made 
in  this  country  as  low  as  01 ;  but  I  am  sure  that  terra  cotta 
is  made  even  as  low  as  cone  05  where  a  slip  and  enamel 
are  applied  to  the  bone  dry  ware  and  matured  in  one 
burn,  which  is  a  much  more  difficult  problem. 

Mr.  Purdy:     Bristol  glaze  type? 

Mr.  8tv.ll:  No.  sir;  an  underglaze  coating  or  slip, 
and  white  tin  enamel  or  glaze  applied  over  it  and  all  ma- 
tured in  one  burn.  If  it  can  be  done  on  large  pieces,  I  do 
not  see  why  it  could  not  be  done  on  small  pieces. 

Mr.  Purdy :     Is  this  over-spray  a  raw  leadless  glaze? 

Mr.  Stull:  No  sir,  a  raw  lead  glaze  not  a  leadless 
glaze. 


12  A    CHEAP    EXAMEL    FOR    STONEWARE. 

Mr.  Purely:  That  is  another  question.  I  do  not  call 
that  a  Bristol  glaze.  We  must  be  careful  to  keep  Bristol 
glaze  apart  from  lead  glazes  because  it  has  different  prop- 
erties. 

Mr,  Jackson  :  The  point  I  was  getting  at  is  how 
large  a  field  the  one-fire  proposition  covers  so  far  as  a 
stoneware  body  is  concerned.  I  do  not  call  any  glaze 
carrying  boric  acid  a  Bristol  glaze. 

Mr.  Purdy:     You  are  wrong. 

Mr.  Jackson  :  That  is  the  idea  I  had.  Is  the  boric 
acid  fritted? 

Mr.  Stall:    Yes,  sir,  it  is  in  series  ten  and  eleven. 

Mr.  Jackson  :  I  would  like  to  ask  Prof.  Orton  if  I 
am  right  in  what  I  said.  When  a  glaze  carries  boric  acid 
fritted  in  it,  can  it  properly  be  called  a  Bristol  glaze? 

Mr.  Orion:  It  does  not  meet  the  idea  of  a  Bristol 
glaze  that  I  have  been  teaching.  A  Bristol  glaze  is  sup- 
posed to  be  a  raw  glaze.  However,  this  glaze  under  dis- 
cussion might  be  a  type  which  we  could  afford  to  use  in 
place  of  the  old  raw  Bristol  glaze.  I  do  not  know  about 
that. 

Mr.  Jackson  :  That  is  as  I  understood  it.  The  glaze 
at  the  top  of  the  list  on  the  blackboard  is  of  the  Bristol 
glaze  type  as  I  understand  it. 

Mr.  Orton  :  It  is  a  little  too  high  in  silica  for  a 
typical  Bristol  glaze. 

Mr.  Stall:  I  am  afraid  a  great  many  are  of  the 
opinion  that  this  work  is  along  the  line  of  Bristol  glazes. 
That  is  not  the  line  of  inquiry,  but  a  cheap  enamel  for 
stoneware  as  indicated  by  the  title  on  the  program.  The 
main  object  of  the  investigation  is  to  produce  a  cheap 
enamel  for  the  lower  grades  of  ware  which  will  be  a  fairly 
good  substitute  for  tin  enamel ;  not  only  for  stoneware 
but  for  other  types  of  ware  maturing  at  lower  tempera- 
tures. That  does  not  confine  the  investigator  to  one  type 
of  glaze.  It  is  a  broad  field  to  work  in.  I  am  sorry  1 
have  not  been  able  to  do  more  work  in  this  line  of  experi- 
mentation. 


A    CHEAP    ENAMEL    FOR    STONEWARE.  13 

Mr.  Fuller:  What  do  you  mean  by  "dope"  to  keep 
the  glaze  from  crawling? 

Mr.  Stutt:  Nearly  all  stoneware  glazes  have  a  ten- 
dency to  crawl  or  bead  because  they  are  highly  viscous. 
In  order  to  prevent  this  crawling  it  is  customary  for  pot- 
ters to  use  such  materials  as  plaster  of  paris,  borax,  car- 
bonate of  soda,  or  some  other  soluble  salt  in  their  glazes. 
Exactly  what  action  these  soluble  salts  have  upon  the 
glaze  has  not  been  definitely  determined.  My  opinion  is 
that  in  the  case  of  borax  and  carbonate  of  soda,  these  ma- 
terials not  only  serve  to  hold  the  glaze  to  the  bone  dry 
body  after  dipping  but  also  fuse  in  the  early  stage  of  burn- 
ing forming  a  sticky  matrix  which  prevents  the  glaze  from 
curling  up,  which  precedes  beading. 

Mr.  Stephani:  I  would  like  to  say  that  I  have  had 
some  experience  in  the  terra  cotta  line  with  beading  and 
crawling,  and  my  impression  is  that  there  are  two  kinds. 
We  get  one  by  getting  the  slip  or  glaze  too  heavy  on  the 
ware  and  the  consequence  is  one  <>r  the  other  cracks  in 
drying  before  it  goes  into  the  kiln,  and  when  the  glaze 
begins  to  fuse  it  beads  and  crawls;  and  in  the  other  case 
we  get  an  effect  which  does  not  appear  before  it  goes  into 
the  kiln.  I  think  manufacturers,  as  a  rule,  for  the  first 
kind  of  beading,  use  a  material  something  like  calcium 
sulphate,  and  some  use  glue  to  hold  the  glaze  together  in 
drying,  making  it  tougher.  For  the  second  kind,  the  kind 
happening  in  the  kiln  and  which  does  not  show  up  in  the 
drying,  boracic  acid  is  used;  or,  in  a  terra  cotta  glaze  the 
ZnO  is  reduced  with  MgO.  The  latter  fault  is  due  to  im- 
perfect glaze,  while  the  former  can  be  overcome  mechanic- 
ally, i.  e.  by  applying  the  underslip  or  glaze  thinner. 

Mr.  Fackt :  May  I  ask  Mr.  Stephani  whether  he  has 
used  Cornwall  stone  in  place  of  the  feldspar  to  prevent 
the  crawling  and  cracking  which  he  speaks  of  as  happen- 
ing before  the  ware  goes  into  the  kiln? 


14  A    CHEAP    EXAMEL    FOR    STOXEWARE. 

Mr.  Stephani:  I  never  used  Cornwall  stone  in  any 
glaze;  but  I  find  the  best  way  to  overcome  that  effect  is 
by  remedying  the  underslip,  or  putting  less  of  it  on  the 
ware. 

Mr.  Miller:  Mr.  Stull  tells  us  he  finds  an  improve- 
ment in  color  by  substituting  magnesium  for  tin. 

Mr.  Purely:  The  substitution  of  magnesium  for  what? 

Mr.  Stull:  An  improvement  in  color  by  substituting 
magnesium  oxide  for  zinc  oxide.  It  does  make  a  little 
difference  in  the  fusibility.  Glazes  containing  MgO  are 
not  quite  so  fusible  as  glazes  containing  ZnO,  other  things 
being  equal.  Dr.  Sinionis,  in  his  work  on  cones,  in  which 
he  used  magnesium  oxide,  points  out  that  the  action  of 
the  MgO  in  the  cones  is  to  hold  the  cones  up  to  a  certain 
point  when  the  fusion  is  comparatively  sudden.  It  is 
claimed  for  that  reason  that  they  are  better  indicators 
of  temperature  than  the  regular  Seger  cones.  We  are 
getting  more  light  all  the  time  as  to  the  action  of  MgO 
in  ceramic  mixtures.  In  fact,  we  do  not  yet  know  all  of 
its  possibilities.  The  magnesite  which  I  used  was  rather 
a  pure  variety  obtained  from  the  Foote  Mineral  Company. 
I  am  told  that  a  good  variety  of  the  Grecian  magnesite  is 
delivered  in  New  York  for  about  twenty-four  dollars  a 
ton.  If  it  can  be  used  successfully  it  will  cheapen  enamels 
wonderfully  and  fill  a  long  felt  want  for  the  lower  grades 
of  ware. 

Mr.  Frink :  One  thing  has  occurred  to  me  in  regard 
to  this  discussion  of  magnesia.  Magnesia  is  used  in  glass 
to  some  extent,  usually  being  found  as  an  impurity  in 
the  lime,  and  experiments  have  been  made  which  show 
that  the  viscosity  is  very  much  reduced  thereby. 

Mr.  Purely:     The  viscosity  is  reduced? 

Mr.  Frink:  Yes.  That  bears  out  what  the  gentle- 
man has  said.  When  the  temperature  rises  to  a  certain 
point,  liquefaction  is  reached  more  rapidly.  Does  not  the 
similar  action  of  borax  account  for  the  beneficial  results 
of  this  material  in  the  case  of  crawling  or  beading,  be- 
cause of  the  filling  of  the  interstices  with  liquid? 


A    CHEAP    ENAMEL    FOR    STONEWARE.  15 

Mr.  Purdy.  Was  it  used  in  a  small  pot  or  a  big 
batch? 

Mr.  Frink:  I  have  experimented  with  it  in  tank  of 
capacity  of  600  tons,  and  also  with  quantities  of  fifty  or 
sixty  pounds. 

Mr.  Purdy:  The  viscosity  seemed  to  be  the  same  in 
both  instances? 

Mr.  Frink :     Yes. 

Mr.  Purdy:  What  was  the  effect  of  magnesia  on  de- 
vitrification? 

Mr.  Frink:  That  seems  to  vary  according  to  con- 
ditions, under  which  it  is  melted.  If  melted  under  a  high 
reducing  flame  devitrification  takes  place  slowly;  but 
under  a  high  oxidizing  flame  it  takes  place  rapidly. 

Mr.  Purdy:  Is  that  peculiar  of  your  magnesium 
glasses  or  is  it  true  also  of  lime  glasses? 

Mr.  Frink:  Yes;  it  is  also  true  as  to  lime  glasses; 
but  with  the  magnesium  glasses  it  is  particularly  so  and 
I  attribute  it  to  the  more  liquid  condition.  That  is  my 
theory. 

Mr.  Purdy:  We  have  to  have  equivalent  of  devitri- 
fication to  have  opacity. 

Mr.  Frink  :  Yes;  and  I  imagine  if  you  put  that  under 
oxidizing  conditions  you  will  get  that  more  rapidly. 

Mr.  Purdy:  I  will  ask  Mr.  Stull  if  magnesium  in  the 
form  of  light,  fluffy  oxide  has  the  same  effect  as  when  in- 
troduced as  magnesite? 

Mr.  Stull:  I  have  never  used  the  light  commercial 
magnesia  only  in  a  fritt  on  one  or  two  occasions,  and  have 
never  made  any  comparisons  between  it  and  the  mineral 
magnesite.  Commercial  magnesia  is  comparatively  ex- 
pensive. We  can  obtain  magnesite  which  will  analyze 
very  high  in  MgC03,  with  very  little  impurity  which  would 
cause  discoloration.  There  is  a  deposit  in  California 
which  is  claimed  by  some  to  be  better  than  the  Grecian, 
but  the  cost  of  mining  it  and  shipping  it  east  is  greater. 
By  a  little  sorting,  a  comparatively  pure  grade  can  be 
had  as  low  as  thirty  to  fortv  dollars  a  ton. 


16  A    CHEAP    ENAMEL   FOB    STONEWARE. 

Mr.  Purdy:  I  asked  that  question  in  the  interest  of 
Hottinger,  Barringer,  and  Hull. 

Mr.  Stall:  What  is  the  effect  of  magnesia  upon  iron 
color  in  glass?  In  series  twelve  and  thirteen  Tennessee 
ball  clay  was  used,  yet  the  glazes  do  not  show  any  trace 
of  cream  color. 

Mr.  Frink :  It  is  so  contradictory  in  action  that  I 
cannot  answer  the  question  definitely.  We  sometimes  get 
one  effect  and  sometimes  another.  I  think  this  is  a  mat- 
ter of  oxidizing  and  reducing  conditions.  It  seems  to  me 
you  would  get  better  results  if  steam  were  thrown  into 
the  kiln.  It  seems  you  have  a  condition  analagous  with 
that  prevailing  in  a  lime  kiln.  I  made  some  experiments 
one  time,  introducing  steam  into  a  lime  furnace.  It  was 
found  that  devitrification  was  very  much  enhanced.  There 
are  two  forms  of  devitrification,  one  due  to  pressure  and 
the  other  to  oxidation.  Pure  oxygen  will  produce  de- 
vitrification. 

Mr.  Purdy.  I  note  that  Mr.  Stull  did  not  draw  any 
conclusions  in  regard  to  soda  or  potash  feldspar,  as  fluxes 
in  these  glazes. 

Mr.  Stull:  As  yet  no  experiments  have  been  made 
in  this  line  to  determine  that.  This  will  probably  be  in- 
vestigated later. 


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